As fossil fuels become more scarce, the energy industry has developed more sophisticated techniques for extracting fuels that were previously too difficult or expensive to extract. One such technique includes injecting steam into an oil-bearing formation to free up the oil. For example, steam can be injected into an oil well and/or in the vicinity of the oil well. The high temperature of the steam heats up the adjacent formation and oil within the formation, thereby decreasing the viscosity of the oil and enabling the oil to more easily flow to the surface of the oil field. To make the process of oil extraction more economical, steam can be generated from solar power using, for example, solar power systems with concentrators (e.g., mirrors) that direct solar energy to a receiver (e.g., piping that contains a working fluid). The concentrators focus solar energy from a relatively large area (e.g., the insolated area of the mirror) to a relatively small area of the receiver (e.g., axial cross-sectional area of a pipe), thereby producing a relatively high energy flux at the receiver. As a result, the working fluid changes its phase (e.g., from water to steam) while flowing through the receiver that is subjected to a high energy flux. Generally, a steady supply of steam is preferred at an oil field for a steady production of oil. However, the production of steam by solar concentrators is a function of solar insolation, which is intrinsically cyclical (e.g., day/night, sunny/cloudy, winter/summer, etc.). Therefore, in some field applications, the solar power systems include solar heat storage devices that can store excess energy when the insolation is high and release energy when the insolation is small or nonexistent. An example of such a system is described below.
FIG. 1 is a schematic view of a system 10 for generating steam in accordance with the prior art. In the illustrated system, the sun 13 emits solar radiation 14 toward a curved concentrator (e.g., a mirror) 11 that has a line focus corresponding to the location of a receiver 12. As a result, the solar radiation 14 from a relatively large curved concentrator 11 is focused on a relatively small area of the receiver 12. As water W flows through the receiver 12, the highly concentrated solar energy causes a phase change from water W to steam S. A first portion of the steam (S1) is directed to an oil well 18 or its vicinity and a second portion of the steam (S2) is directed to a heat exchanger 15. A valve V maintains a suitable balance between the flows of steam S1 and S2. For example, the valve V can be fully closed when the steam production is relatively low, and all available steam is directed to the oil well 18. When there is excess steam available (e.g., during a period of high insolation), the second portion of steam S2 enters the heat exchanger 15, exchanges thermal energy E with a working fluid WF, which can be, for example, steam or thermal oil, and returns to the entrance of the receiver 12. Depending on the exchange of energy E in the heat exchanger 15, the temperature of the second portion of steam (S2) may still be higher than that of the water W, thereby decreasing the amount of solar energy that the water W would otherwise require to change its phase to steam.
As explained above, when the insolation is relatively high, the temperature of the second portion of steam (S2) is sufficiently high to transfer thermal energy to the working fluid WF in the heat exchanger 15. The working fluid WF then transfers thermal energy to a heat storage unit 16. Conversely, when the insolation is relatively low, the temperature of the second portion of steam (S2) is also relatively low, and the second portion of steam (S2) receives thermal energy from the working fluid WF in the heat exchanger 15. Overall, thermal energy that is stored in the heat storage device 16 when the insolation is relatively high is transferred back to steam when the insolation is relatively low. This transfer of thermal energy to and from the heat storage device 16 promotes a more even flow of the first portion of steam S1 at the oil well 18. Some examples of the prior art heat storage devices are described in the following paragraphs.
FIG. 2 illustrates a portion 20 of a heat storage device in accordance with the prior art. In the portion 20 of the heat storage device (e.g., the heat storage device 16 of FIG. 1), concrete blocks 22 surround pipes 21. When the temperature of the working fluid WF is relatively high, the flow of the working fluid WF through the pipes 21 heats up the adjacent concrete blocks 22. This part of the thermal cycle generally occurs during a period of high insolation. Conversely, when the insolation is low, the concrete blocks 22 heat the working fluid WF, which then transfers energy back to the water/steam in the heat exchanger 15 (FIG. 1). Accordingly, the heat storage device 16 recovers some thermal energy that would otherwise be wasted due to the cyclical nature of insolation. However, the illustrated system has some drawbacks. For example, the pipes 21 are relatively expensive, making the overall heat storage device 16 expensive. Due to a relatively dense distribution of the pipes 21, the amount of working fluid WF contained in the heat storage device 16 can be relatively high which further increases cost of the heat storage device 16. Furthermore, the rate of heat transfer can be poor at the junction between the pipes 21 and the concrete blocks 22, therefore reducing the efficiency of the heat storage process.
FIG. 3 is a partially schematic cross-sectional view of another heat storage device 30 in accordance with the prior art. A first working fluid WF1 (e.g., steam or oil) flows through a piping system 33 and exchanges thermal energy with a second working fluid WF2 (e.g., oil) contained in the heat storage device 30. The second working fluid WF2 can be heated by the first working fluid WF1 during periods of high insolation and the first working fluid WF1 can be heated by the second working fluid WF2 during periods of low insolation. In general, the second working fluid WF2 can absorb relatively large amount of heat without having to be pressurized due to its relatively high heat capacity and boiling point. Because the second working fluid WF2 is generally expensive, relatively inexpensive concrete plates 31 can be inserted in the heat storage device 30 to reduce the required volume of the second working fluid WF2 inside the heat storage device 30. To improve the heat transfer to/from the concrete plates 31, pumps 32 circulate the second working fluid WF2 within the heat storage device 30. However, the flow of the second working fluid WF2 around the concrete plates 31 can still vary significantly, resulting in thermal non-uniformities when heating/cooling the concrete plates 31, thereby reducing the thermal capacity of the system. Furthermore, the pumps 32 are potential points of failure within the overall system. Accordingly, there remains a need for inexpensive and thermally efficient heat storage devices that can facilitate solar heat storage and recovery.